This invention relates generally to balanced amplifiers and more particularly to transistors utilized in such amplifiers.
Gallium arsenide (GaAs) field-effect transistors (hereafter FETs) when used as active elements in power amplifiers exhibit parasitic loss mechanisms which adversely affect the amplifier gain. These loss mechanisms are associated with the source electrode of the FET and include (1) the channel resistance r.sub.c between the gate electrode and the source electrode, (2) the resistance R.sub.c of the source metallization contact, (3) the resistance R.sub.s of the source lead wire, and (4) the inductance L.sub.s of the source lead wire. In a class A power FET amplifier, signal current at the fundamental frequency flows in the lossy components r.sub.c, R.sub.c, R.sub.s and L.sub.s. These parasitic loss mechanisms can account for up to 6 db of loss at microwave frequencies. Since a typical class A power FET amplifier exhibits less than 5 db of net gain at its rated output power, the loss is significant.
A class B push-pull amplifier employing two FETs requiring two input signals 180.degree. out of phase with each other, and a center-tapped output transformer, has been disclosed in the paper "X-Band Monolithic GaAs Push-Pull Amplifiers", V. Sokolov et al, 1979 IEEE International Solid-State Circuits Conference at page 118. Each FET exhibits the loss mechanisms r.sub.c, R.sub.c, R.sub.s and L.sub.s associated with its source terminal and signal currents at the fundamental frequency flow in the lossy components. However, by placing the two FETs side-by-side on the same GaAs chip and connecting the source lead wires together, the source lead-wire resistance R.sub.s and the source lead-wire inductance L.sub.s are significantly reduced. In the aforementioned class B push-pull amplifier, the improved gain was found to just exceed the initial losses associated with splitting the input signal from an unbalanced to a balanced configuration. Nevertheless, at frequencies above 15 GHz, the lossy components r.sub.c, R.sub.c, R.sub.s and L.sub.s continue to further degrade amplifier gain and efficiency.